The present invention relates to heat cooking apparatuses, and, more particularly, to a heat cooking apparatus such as an electric oven or a microwave oven with an electric heater wherein food in a heating chamber is heated by a heating device.
In a conventional electric oven or a microwave oven with an electric heater, heating devices have been adopted with a heating element shaped as a metal, commonly called a sheathed heater, or with a flat heating element sandwiched with flat insulation sheets, commonly called a flat heater. Flat heaters are roughly divided into two types; wall types and built-in types. A wall type flat heater is installed into an opening which is provided in the heating chamber wall, while a built-in type flat heater is installed within a predetermined space in the heating chamber, several examples of conventional heating devices being shown in FIGS. 1 through 4.
FIG. 1 is a cross-sectional view showing a conventional microwave heating range with build-in type sheathed heaters, and FIG. 2 is a perspective view showing the sheathed heater shown in FIG. 1. As shown in FIG. 1, the heating chamber 1 comprises an upper heater 2, a lower heater 3, and a pan 4 where food material 5 to be heated and cooked is placed. Furthermore, the apparatus is provided with a magnetron 6 which irradiates microwaves into the heating chamber 1 through a waveguide 7 to heat food 5. Thus, FIG. 1 shows a so-called compound-heating oven cooking range using electric heaters and microwaves, and the heater used in the heat cooking apparatus of FIG. 1 is shown in FIG. 2.
In this heat cooking apparatus of conventional construction, the upper heater 2 and lower heater 3 are exposed in the heating chamber 1, so that the effective capacity of the heating chamber 1 is reduced due to a reduction in volume of the heater. In order to contain large-sized food material, conventional heating chambers must be made larger, and, consequently, the external dimension of the conventional apparatus is proportionally larger requiring a larger space for it to be place, thus making it inconvenient to use. Moreover, heaters exposed in the heating chamber make it difficult to clean inside surfaces of the heating chamber soiled with scattered food material, making it even more inconvenient to use.
More specifically, the lower heater 3 is detachable so that the bottom face of the heating chamber can be easily cleaned when food material or soup drips on the face. However, to prevent microwave leakage, the joining part of the lower heater 3 to the heating chamber 1 is very complicated. The upper portion of the heating chamber 1 is easily stained and difficult to clean, even though the upper heater 2 is undetachable. Accordingly, this portion of the heating chamber is provided with a so-called self-cleaning layer which has a self-cleaning function to decompose adhered oil stains into water as well as carbon dioxide gas at temperatures higher than a predetermined temperature.
Even when this apparatus is used for grill cooking, where the upper heater reaches an allowable highest temperature, the temperature at the self-cleaning layer confronting the upper heater 2 scarcely reaches 300.degree. C., so that the self-cleaning layer cannot perform total self-cleaning. In fact, this apparatus is commonly used in homes for oven cooking bread or cake, not for grill cooking food. When this apparatus is used for oven cooking, the large quantity of electric power supplied to the lower heater 3 at the bottom results in a lower temperature on the self-cleaning layer at the upper portion of heating chamber, supressing its self-cleaning potential. In addition, since the upper and lower heaters 2 and 3 are exposed in the heating chamber 1, the food material 5 directly receives radiation heat, especially from the upper heater 2, which locally burns the food in the pattern of the upper heater 2.
Although the lower heater 3 is the assembly or disassembly of it is troublesome. If the lower heater 3 is removed and washed with water, trouble may occur with its insulation or durability. Above of all, the exposure of heaters in the heating chamber 1 not only prevents easy cleaning, but is also unaesthetic.
FIG. 3 is a perspective view showing a conventional heat cooking apparatus with a wall type flat heater, wherein a part of the ceiling of the heater chamber is recessed to form an opening and a heater formed as a flat sheet is inserted and installed into the opening. In this construction, a significant thermal gradient is produced at the joining portions between the flat heater 8 and the ceiling of the heating chamber 1. That is, the temperature of the heater 8 rises quickly when energizing starts, and the heater 8 expands in a planar direction. The adjacent ceiling area of the heating chamber 1, however, remains at room temperature so that significant mechanical stress occurs at the joining portions between them. If this phenomenon is frequently repeated, a crack will appear at the joining portion which results in damage, and especially, in the case where microwave heating is jointly used, microwaves may leak or sparks may emit from said cracks. In this construction, it is difficult to provide a flat heater 8 over the entire ceiling area of the heating chamber 1, and, consequently, uneven heating by the heater cannot be totally eliminated.
Further, an example of a conventional heating apparatus, with a built-in type flat heater 8 provided in the heating chamber 1, is shown in FIG. 4. This construction possesses all the disadvantages common to the wall type flat heater 8 described above. In addition, the built-in flat heater 8 also prevents easy cleaning of the inside of the heating chamber, and significantly reduces the effective capacity of the heating chamber 1.